Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communications systems. For example, long-haul transmission networks employ single-mode optical fibres for the transmission of digital information at very high bit rates, using one or more optical carriers, or wavelengths, over each fibre. Furthermore, optical transmission over shorter distances, and at lower bit rates, such as in customer access networks or local area data networks, are known which employ either single-mode or multi-mode optical fibres. Additionally, free space optical transmission paths may be used for line-of-sight communications and/or to provide a simple means of wireless interconnection of devices such as computers and peripherals, Personal Digital Assistants (PDAs), and other portable devices, typically using infrared light sources and detectors.
Accordingly, there is an ongoing interest in providing optical communications methods and devices that enable such communications to be conducted over greater transmission distances, and/or with greater reliability, efficiency and/or capacity.
However, optical transmission paths, whether over single-mode or multi-mode optical fibres, or through free space, may introduce distortion in the transmitted optical signals. Furthermore, the devices used for the transmission and reception of optical signals are possible additional sources of signal distortion. A common form of distortion is a frequency-dependent variation in the response of the communications channel, which may result in suppression of the amplitude of the signal (or fading) over particular frequency ranges and/or at specific frequencies, as well as variations in delay, or phase, of the received signal as a function of frequency. Such distortion may result from the frequency characteristics of the various electronic and optical devices used in the transmission system, as well as the characteristics of the optical transmission path itself.
In particular, the various types of optical path generally exhibit forms of dispersion, characterised by variations in the delay experienced by optical signals, or components thereof, transmitted over the optical path. Free space optical paths may exhibit multi-path effects, in which signals transmitted from an optical source reach a remote optical receiver via a number of different spatial paths, which may include a direct (line-of-sight) path as well as one or more paths including reflections off various surfaces in the surrounding environment. Accordingly, different components of the received optical signal experience different transmission delays, resulting in a time spread in the received optical signal which is one form of dispersion.
In multi-mode optical fibres, signals coupled into a fibre excite a large number of transverse optical modes supported by the fibre, each of which is characterised by a different group velocity. Accordingly, the components of the transmitted signal coupled into different transverse modes experience differing transmission delays along the multi-mode fibre, again resulting in dispersion of the received signal.
The dispersion resulting from multi-path propagation in free space optical transmission systems, and from modal dispersion within multi-mode fibres, may be either relatively static or time-varying, and can result in corresponding static or time-varying frequency fading in the received signal.
In single mode optical fibres, no multi-path or modal dispersion exists, however, in general, signals transmitted through single-mode fibres will experience various forms of degradation including chromatic dispersion, according to which different frequency components of the signal propagate at different speeds, and polarisation mode dispersion (PMD), according to which components of the signal coupled into different polarisation states propagate at different speeds. Both of these dispersion mechanisms, amongst others including various nonlinear processes, may result in spreading of transmitted pulses.
Additionally, all optical transmission channels will exhibit other forms of frequency-dependent distortion resulting from the characteristics of the transmitting and receiving electronic and opto-electronic components.
Methods for mitigating, or compensating, frequency-dependent distortion in communications channels are known, and have previously been extensively applied in radio frequency (RF) communications systems, including both wireless and wire-line systems. A particularly effective method for mitigating the detrimental effects of RF communications channels involves encoding information for transmission into discrete blocks of data which are then transmitted through the communications channel. At the receiver, each block is recovered, and an equalisation function performed, preferably in the frequency domain, in order to reduce, or eliminate, the frequency-dependent effects introduced in the channel. Many such methods employ some form of multi-carrier modulation, in which a serial data stream is converted into blocks consisting of several parallel data streams, which are then transmitted on separate frequency sub-carriers. At the receiving end, each of the parallel data streams is received, equalised according to the channel frequency response, and decoded to recover the original serial data stream provided to the transmitted.
Amongst the more popular forms of multi-carrier modulation methods is orthogonal frequency division multiplexing (OFDM). In OFDM systems, each sub-carrier is sinusoidal, and the particular set of sub-carriers is chosen such that all are orthogonal with one another over each transmitted symbol period. In a particularly convenient implementation, the required orthogonality is achieved through the use of orthogonal transforms in the transmitter and receiver respectively, and in the particular case of OFDM discrete Fourier transforms are used.
Modulation methods of the type described in the foregoing, including multi-carrier modulation schemes such as OFDM, offer several advantages over transmission using a single carrier modulated at a high modulation rate. These include the ability to operate over dispersive channels with complex time spreading, phase versus frequency and amplitude versus frequency characteristics. Accordingly, it would appear desirable to utilise such modulation methods over dispersive optical communications channels, such as free space channels, multi-mode fibres and single-mode fibres.
However, the use of such modulation schemes over optical channels has, to date, proven problematic. The aforementioned modulation methods, which have proven highly effective in RF communications systems, all involve the generation and transmission of bipolar signals, that is signals having both positive and negative excursions in amplitude. Bipolar signals are readily generated and transmitted in RF systems, in which the carriers are electromagnetic fields generated and detected as time-varying voltages and currents, which may take on arbitrary positive or negative values in accordance with the desired signal amplitude.
However, the simplest and most readily implemented form of optical modulation is Intensity Modulation (IM), in which the transmitted signal amplitude is represented by instantaneous optical power. This is inherently a unipolar modulation system, since negative values of power or intensity are not physically meaningful, and cannot be generated.
Past efforts to overcome the limitations inherent in unipolar intensity modulation have proven largely ineffective or impractical. One solution is to apply a relatively high bias level at the transmitter, such that the positive and negative excursions in transmitted signal amplitude are represented by variations around a fixed average optical output power of the transmitter. However, this is highly inefficient in its utilisation of the available optical transmission power, particularly in the case of multi-carrier modulation schemes which may generate signals having a very large peak-to-average power ratio, such that the peak excursions are much larger than the signal average value. While optical power inefficiency may be an issue in all types of optical transmission systems, an additional problem in free space systems is that the required optical output power may exceed eye-safe levels.
Other methods for encoding bipolar signals onto an optical channel include the use of more complex optical modulation schemes, such as multiple optical carriers with differential modulation, or the use of frequency, phase or polarisation modulation methods. However, all of these methods result in additional cost and complexity, while some require the use of coherent optical channels, and as such they are generally too expensive or otherwise impractical for many relevant applications. It would therefore be desirable, and is accordingly an object of the present invention, to provide methods and apparatus that employ suitable modulation and equalisation schemes for compensation of optical channel characteristics, and in particular the effects of various types of dispersion occurring in optical channels, while mitigating the aforementioned disadvantages of known optical modulation methods when applied to bipolar signals.